Lake Tahoe warmer, dirtier - scientists worry

Published 4:00 am, Thursday, August 16, 2007

Lake Tahoe, the jewel of the Sierra once so crystal-clear that Mark Twain likened boating on it to floating on air, is warmer and soupier than ever before as a result of climate change and human activities, UC Davis scientists reported Wednesday.

Their 45-page report, the most comprehensive ever done on the lake, outlines significant changes in weather patterns over the years, including less snowfall and more rain, deteriorating lake clarity and increasing water temperature in the Lake Tahoe Basin - all of which could increase invasions of exotic fish and plant species.

"Change is a difficult thing, and the lake is changing," said Geoff Schladow, director of the UC Davis Tahoe Environmental Research Center and co-editor of the report. "But I think it is too early to say that our efforts are in vain. We can't judge each year in isolation. It's not all doom and gloom."

But there is reason to be concerned about the second-deepest lake in the United States, researchers said. Conditions appear to be getting worse, even as environmental and planning agencies work to reduce runoff from residential and commercial development and improve water quality in the lake.

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The most significant finding, according to Schladow, is how much the Tahoe climate is warming. Low temperatures at night have risen 4 degrees on average, and the number of days with temperatures that averaged below freezing dropped from 79 days to 52 days since 1910.

The percentage of precipitation that falls as snow has also decreased, from 52 percent to 34 percent in the 96 years studied.

All of this is apparently having a major effect on the lake. The average temperature of the surface water in July has increased almost 5 degrees, from 62.9 degrees to 67.8 degrees since 1999, according to the report. The water temperature was 78 degrees on July 26, 2006, the warmest in Lake Tahoe's recorded history.

Clarity also has suffered. In 2006, Lake Tahoe was clear to an average depth of 67.7 feet, based on how far from the surface researchers could see a white dinner-plate-size measuring tool known as a Secchi disk.

That's 4.6 feet less than 2005. When measurements began in 1968, the lake was clear to an average depth of 102.4 feet. That's how clear the country's deepest lake, Oregon's Crater Lake, is today, Schladow said.

It is impossible to say what the Secchi disk measurement would have been when Mark Twain visited Lake Tahoe in the 1860s, but it was nothing short of astonishing to him.

"So singularly clear was the water, that where it was only twenty or thirty feet deep the bottom was so perfectly distinct that the boat seemed floating in the air!" Twain wrote in "Roughing It." He said even below 80 feet the water was as clear as glass.

"Every little pebble was distinct, every speckled trout, every hand's-breadth of sand," he wrote. "Down through the transparency of these great depths, the water was not merely transparent, but dazzlingly, brilliantly so."

UC Davis experts say the clarity has deteriorated because fine particles from erosion, urban runoff and pollution have entered the lake. The particles fuel the growth of algae, which absorb light and increase temperature.

Schladow said invasive species of fish have been in the lake for about 10 years, but they mostly hang around marinas. The concern is that warming water will create better habitat throughout the lake for these species.

"The persistent increase in water temperature that we have observed since 1978 is beginning to have noticeable impacts on the entire Lake Tahoe ecosystem," said Schladow. "The types of algae we see in the lake are changing, and they are starting to be present earlier in the year. The lake is becoming more hospitable to invasive plants and fish, with warm-water species like bass and carp increasingly common."

Julie Regan, communications and legislative affairs chief for the Tahoe Regional Planning Agency, said invasive species are a major worry. The agency, which is in charge of land use and environmental planning for the Tahoe basin, declared a state of emergency this year after a Quagga mussel was found by agricultural inspectors attached to a boat that was being transported to Lake Tahoe.

The Quagga is similar to the Zebra mussel, an invasive species that has been wildly multiplying with devastating effects throughout the United States.

"It's certainly a concern," Regan said. "How this relates to climate change is something we really need to discuss and pay attention to."

The regional planning agency has been working closely with UC Davis and other research institutions and agencies in an attempt to preserve the Tahoe ecosystem. They developed the Lake Tahoe Environmental Improvement Program 10 years ago.

The organizations are developing plans to reduce commercial and household runoff into the lake and restore water quality with the help of state forestry officials in California and Nevada. Officials have said they intend to bring water clarity in the lake back up to 100 feet.

"Our environmental goals are long range," Regan said. "We know that some of our goals - lake clarity for instance - may take many generations to achieve."

Schladow said such goals will be more difficult as new homes and businesses, parking lots and roadways continue to be built in the region. The Angora Fire, which burned 3,072 acres in South Lake Tahoe earlier this summer, destroying 254 homes and causing an estimated $153 million of damage, didn't help matters.

"The majority of the runoff comes from where we are, these urban areas," he said.

Schladow said the State of the Lake Report will be released annually to give the public a better understanding of the changes occurring in the basin and help scientists compile long-term data on whether cleanup programs or regulatory efforts are working.

He does not believe it is too late to restore Lake Tahoe to what it once was, but he said it will be a difficult task.

"It's something the scientists and the stakeholders in the Tahoe basin will need to address," Schladow said. "It is going to take a lot of effort and a lot of money to do that. Society will have to decide whether it is worthwhile."

For more information on the Tahoe Regional Planning Agency, go to:

Lake Tahoe's changes

With a maximum depth of 1,645 feet, Lake Tahoe is the second-deepest lake in the United States (behind Oregon's Crater Lake) and is known for its cool, pristine waters. But it has become warmer and less clear over the years, scientists say:

-- Surface water temperatures in July have increased 5 degrees. in the past seven years, with the warmest average temperature in July 2006 of 67.8 degrees.

-- The number of days with average air temperatures below freezing at lake level has decreased by 27 days a year since 1910, while the percentage of snow in total precipitation has dropped from 52 percent to 34 percent since 1911.

-- The distance below the lake surface at which a 10-inch white disk cannot be seen has been decreasing. In 1968, the distance was about 100 feet; last year, it was 67.7 feet.